1. Field of the Invention
This invention relates to a process for preparing a radiation image storage panel which is employable in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of the Prior Art
For obtaining a radiation image, there has been heretofore employed a radiography utilizing a radiographic film having a photosensitive silver halide emulsion layer in combination with a radiographic intensifying screen.
As a method replacing the above-mentioned radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for example, in U. S. Pat. No. 4,239,968, has been utilized. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having been radiated by an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (namely, stimulating rays) to sequentially release the radiation energy stored in the stimulable phosphor as light emission (stimulated emission); photo-electrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the above-described radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably small dose, as compared with the case of using the conventional radiography. Accordingly, this radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the radiation image recording and reproducing method has a basic structure comprising a support and a stimulable phosphor-containing resin layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the stimulable phosphor-containing resin layer to keep the stimulable phosphor-containing resin layer from chemical deterioration or physical shock.
The stimulable phosphor-containing resin layer generally comprises a resinous binder and stimulable phosphor particles dispersed therein. The stimulable phosphor particles in the stimulable phosphor-containing resin layer, when excited with stimulating rays after having been exposed to a radiation such as X-rays, emit light (stimulated emission). Accordingly, the radiation having passed through an object or having been radiated by an object is absorbed by the stimulable phosphor-containing resin layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the radiation image storage panel in the form of a radiation energy-stored image (in the form of a latent image). The radiation energy-stored image can be released as stimulated emission (in the form of light emission) by applying stimulating rays to the panel, for instance by scanning the panel with stimulating rays. The stimulated emission is then photo-electrically converted to electric signals, so as to produce a visible image from the radiation energy-stored image.
Accordingly, it is desired for the radiation image storage panel employed in the radiation image recording and reproducing method to have a high sensitivity and to provide an image of high quality (high sharpness, high graininess, etc.). In particular, from the viewpoint of obtaining more accurate and detailed information of an object, it is desired to develop a radiation image storage panel which provide an image of improved sharpness.
The sensitivity of the radiation image storage panel is essentially determined by the total amount of stimulated emission produced by the stimulable phosphor contained therein, and the total amount varies depending upon not only the emission luminance of the phosphor but also the content (i.e., amount) of the phosphor in the phosphor layer. The large content of the phosphor also results in increase of absorption of a radiation such as X-rays, so that the panel shows an increased high sensitivity and provides an image of improved quality, especially, improved graininess. On the other hand, assuming that the content of the phosphor layer is kept at the same level, a panel utilizing such a phosphor layer provides an image of high sharpness, if the phosphor layer is densely packed with the phosphor, because such phosphor layer can be made thinner to reduce spread of stimulating rays which is caused by scattering in the phosphor layer.
U.S. Pat. No. 4,910,407 describes a radiation image storage panel having a phosphor layer in which stimulable phosphor particles are densely packed by compressing the prepared phosphor layer so as to reduce the void volume of the layer.
The phosphor density in the phosphor layer of the above-mentioned panel is increased by compressing the phosphor layer, and an image provided by the panel exhibits improved sharpness as compared with one obtained by a previously known radiation image storage panel. However, with respect to the graininess, the panels sometimes relatively deteriorate.
U.S. patent application Ser. No. 07/510,679, now U.S. Pat. No. 5,161,224 describes a radiation image storage panel showing high sharpness as well as good graininess. The radiation image storage panel utilizes a thermoplastic elastomer as a binder of the stimulable phosphor layer and is prepared by compressing a phosphor layer on a support at a temperature of not lower than softening point or melting point of the thermoplastic elastomer.